This invention relates to a demagnetizing circuit and a current limiting device particularly for use in this demagnetizing circuit.
Conventionally, a power supply circuit of a color television receiver or color monitor display or the like has a built-in demagnetizing circuit for automatically demagnetizing a cathode ray tube. A first circuit construction of this kind of power supply circuit and the demagnetizing circuit is shown in FIG. 9. That is, this power supply circuit has a line filter 2 connected to a commercial alternating current power supply 1, a smoothing capacitor 4 connected to a main circuit 3 and a rectifier diode bridge 5 interposed between the line filter 2 and the smoothing capacitor 4. Between the line filter 2 and the rectifier diode bridge 5 are disposed a main switch 6 and a negative temperature characteristic thermistor (hereinafter called an NTC) 7 for suppressing surge currents to the smoothing capacitor 4. As shown with dotted lines in FIG. 9, the NTC 7 is sometimes disposed between the smoothing capacitor 4 and the rectifier diode bridge 5; also, a fixed resistance (not shown in the drawings) is sometimes used instead of this NTC 7.
A demagnetizing circuit is connected in parallel with the power supply circuit and draws on the commercial alternating current power supply 1 from between the rectifier diode bridge 5 and the main switch 6 and between the line filter 2 and the NTC 7, and comprises a demagnetizing coil 8 for demagnetizing a cathode ray tube (not shown in the drawings), a first positive temperature characteristic thermistor (hereinafter called a PTC) 9 connected in series at one side of the demagnetizing coil 8 and a second PTC 10 connected in parallel between the other side of the demagnetizing coil 8 and the PTC 9. The resistance of the PTC 9 increases as its temperature rises, gradually decreasing a demagnetizing current flowing to the demagnetizing coil 8 and the PTC 10 heats the PTC 9 to reduce the residual demagnetizing current
However, when the main switch 6 is closed in the conventional power supply circuit and demagnetizing circuit described above, the NTC 7 or fixed resistance and the first and second PTCs 9 and 10 give off heat, so that while the main switch 6 is closed about 2 to 3 watts of electrical power are constantly consumed. Consequently, a printed circuit board (not shown in the drawings) on which the various components are mounted is damaged and other components deteriorate. In particular, because this PTC 10 reaches temperatures of over 150.degree. C., its life is unavoidably short.
To avoid these problems, a second circuit construction shown in FIG. 10 comprising a relay circuit interposed between a power supply circuit and a demagnetizing circuit has also been employed. In this conventional construction the PTC 10 is dispensed with. A mechanical relay 11 is connected to one side of the PTC 9, and a demagnetizing current is cut off by the mechanical relay 11 being opened by the operation of a relay driving circuit 12 based on a relay control signal. However, when this kind of construction is employed the relay driving circuit 12 is necessary Thus, not only is the occupied circuit board area increased by the relay driving circuit 12 but also there is the problem that the cost is increased along with the use of the expensive mechanical relay 11.